(05-26) 11:33 PDT SAN FRANCISCO — California voters legally outlawed same-sex marriage when they approved Proposition 8 in November, but the constitutional amendment did not dissolve the unions of 18,000 gay and lesbian couples who wed before the measure took effect, the state Supreme Court ruled today.

The 6-1 decision upholding Prop. 8 was issued by the same court that declared a year ago that a state law defining marriage as the union of a man and a woman violated the right to choose one’s spouse and discriminated on the basis of sexual orientation.

Prop. 8 undid that ruling. The author of last year’s 4-3 decision, Chief Justice Ronald George, said today that the voters were within their rights to approve a constitutional amendment redefining marriage to include only male-female couples.

“All political power is inherent in the people,” George said, quoting the Declaration of Rights in the state Constitution. He said the voters’ power to amend their Constitution is limited – and might not include a measure that, for example, deprived same-sex couples of the right to raise a family – but that Prop. 8 did not exceed those limits.

Under California’s domestic-partner law and anti-discrimination statutes, the chief justice said, “same-sex couples continue to enjoy the same substantive core benefits … as those enjoyed by opposite-sex couples, including the constitutional right to enter into an officially recognized and protected family relationship with the person of one’s choice and to raise children.”

The voters, he said, have added “the sole, albeit significant, exception that the designation of ‘marriage’ is … now reserved for opposite-sex couples.” That was within their authority, George said, and any further change can come only at the ballot box.

In dissent, Justice Carlos Moreno, who joined the majority in last year’s decision, said today’s ruling accepted the separate-but-equal treatment for gays and lesbians that the 2008 ruling rejected.

“Granting same-sex couples all of the rights enjoyed by opposite-sex couples, except the right to call their officially recognized and protected family relationship a marriage, still denies them equal treatment,” Moreno said.

The justices ruled unanimously that Prop. 8 was not retroactive and that gay and lesbian couples who relied on the court’s May 2008 ruling to get married before the Nov. 4 election will remain legally wed.

Denying recognition to those marriages would disrupt “thousands of actions taken in reliance on (last year’s ruling) by these same-sex couples, their employers, their creditors, and many others … potentially undermining the ability of citizens to plan their lives according to the law as it has been determined by this state’s highest court,” George wrote.

Prop. 8, which declared that only marriage between a man and a woman is valid or recognized in California, passed with a 52 percent majority after an intense and expensive campaign. Sponsors, mainly affiliated with Christian conservative groups, raised nearly $40 million for the measure and opponents more than $45 million – combined, a record for a ballot measure on a social issue anywhere in the nation.

The ruling, the court’s third major decision on same-sex marriage in five years, may be the last word from the state’s legal system on the issue. But the matter is far from settled in the political arena. Gay-rights advocates, anticipating the decision, have discussed putting another constitutional amendment on the ballot in 2010 or 2012 to try to repeal Prop. 8.

Meanwhile, same-sex marriage has been legalized by the Supreme Courts of Iowa and Connecticut and the legislatures of Vermont and Maine, joining Massachusetts, whose high court issued the first such ruling in 2003. Similar legislation is pending in New Hampshire and New York.

California’s legal battle dates back to February 2004, when San Francisco Mayor Gavin Newsom authorized the city clerk to issue marriage licenses to same-sex couples. Nearly 4,000 weddings took place in the next month before the state Supreme Court ordered a halt, then voided the marriages in August 2004 and found unanimously that Newsom had no authority to disregard state law.

The city and a number of couples quickly returned to court and sued to overturn the law. They won in Superior Court, lost in an appeals court, and won in the state’s high court on May 15, 2008 – but by then, their opponents had already submitted more than 1 million signatures qualifying Prop. 8 for the November ballot.

This time, the issue before the justices was whether the voters’ power to amend the Constitution by initiative.

Plaintiffs in the lawsuits were two groups of same-sex couples, some already married and some thwarted by Prop. 8, along with an array of local governments led by San Francisco. They argued that a measure eliminating fundamental rights exceeds the scope of a constitutional amendment and amounts to a revision, which needs a two-thirds legislative vote or approval from delegates at a state constitutional convention to reach the ballot.

Attorney General Jerry Brown, who usually defends state laws in court, joined Prop. 8’s opponents and argued that “inalienable rights” in the California Constitution cannot be repealed by majority vote.

Prop. 8’s sponsors noted that the court had declared ballot measures to be revisions only twice. As George pointed out in today’s ruling, the court has rejected similar challenges to such far-reaching measures as a legislative term-limits initiative, the Proposition 13 tax cut and the reinstatement of the death penalty.

To be a constitutional revision, the chief justice said, a measure “must make a far-reaching change in the fundamental governmental structure or the foundational power of its branches.” He said Prop. 8 leaves the governmental structure intact and preserves the judicial power to protect rights under the Constitution.

Moreno, in dissent, argued that the initiative process must not be used to allow a majority to deny fundamental rights to a historically persecuted minority.

The ruling, he said, “is not just a defeat for same-sex couples, but for any minority group that seeks the protection of the equal-protection clause of the California Constitution.”